Follower-type amplifiers are widely used in electronic signal processing circuits, particularly in impedance matching or signal buffering situations. Such follower-type amplifiers, also known as emitter followers, FET source followers, voltage followers, etc., are typically used as the signal input stage for electronic test and measurement instruments, such as oscilloscopes and the like. In the implementation of follower-type amplifiers as input stages, where precise signal replication may be required over a wide frequency range, e.g., from D.C. to a few hundred megahertz, amplifier drift characteristics must be compensated. That is, amplifier stabilization must be provided for performance at the lower frequencies, particularly at or near D.C.
To solve the drift or stabilization problems, many solutions have been offered in which the correction or compensation is attempted on the output side of a particular active device. For example, it is a common practice with field-effect transistor (FET) source follower circuits to provide a second FET with matched characteristics as a current source or a current sink, the effect being that the drift producing mechanisms of the two FETs cancel each other, providing a stabilized output. Such a circuit, however, has a fairly high output impedance, e.g., about 150 ohms, and is subject to thermal drift. The problem increases in complexity when bipolar transistors are added to reduce the output impedance or to provide compensation because of their differing operating characteristics. Here, the attempted solution is to control the FET operating bias by adding an operational amplifier to control the current source or current sink transistor. However, this scheme has its drawbacks as well, causing shifting bias levels throughout the circuit and perhaps any subsequent following stages. It is also difficult to maintain zero D.C. offset from input to output.
Another attempt to solve the stabilization problem involved an operational amplifier which compared the follower output with the input and provided a corrective current through a resistor to the base of the follower transistor. The drawback with this system is that the follower device is fast-acting on high-speed signal transitions while the response of the operational amplifier is slow in the sense that the correction current causes a voltage drop across the aforementioned resistor, causing a shift in the bias conditions of the follower device.